1. Field of the Invention
The present invention relates to a semiconductor device having an overheat and overcurrent protective function.
2. Description of the Related Art
A conventional semiconductor device is described with reference to FIG. 4 which is a circuit diagram illustrating the conventional semiconductor device.
The conventional semiconductor device includes an overcurrent detection unit 304, a thermal shut down detection unit (hereinafter referred to as TSD detection unit) 309, NMOS transistors 301, 302, 306, and 307, resistors 303, 305, and 308, a ground terminal 100, and external terminals 321 and 322. The overcurrent detection unit 304, the resistor 303, and the NMOS transistor 306 form an overcurrent protective circuit 331. The NMOS transistor 307 and the TSD detection unit 309 form an overheat protective circuit 332.
The NMOS transistor 301 is controlled to be turned on/off in response to a signal from the external terminal 322. The overcurrent protective circuit 331 protects the NMOS transistor 301 from overcurrent. Similarly, the overheat protective circuit 332 protects the NMOS transistor 301 from overheat. The overcurrent protective circuit 331 includes the overcurrent detection unit 304. The overcurrent detection unit 304 detects a drain current ID of the NMOS transistor 301 by, for example, referring to current which flows through the NMOS transistor 302. When the current ID reaches an overcurrent limit value, the overcurrent detection unit 304 turns on the NMOS transistor 306 and forcibly grounds the external terminal 322 to turn off the NMOS transistor 301. In this way, the NMOS transistor 301 is protected from breakage due to overcurrent. The overheat protective circuit 332 includes the TSD detection unit 309. When the temperature of the semiconductor device reaches an initially set temperature, the TSD detection unit 309 turns on the NMOS transistor 307 and forcibly grounds the external terminal 322. In this way, the NMOS transistor 301 is protected from breakage due to overheat.
The overcurrent detection unit 304 of the overcurrent protective circuit 331 detects the drain current ID of the NMOS transistor 301. When the current ID reaches an overcurrent detection value, the overcurrent protective circuit 331 exerts control so that the response time of the overheat protective circuit 332 is reduced to inhibit the energy applied to the NMOS transistor 301. In this way, in an area in which the apparent allowable power is high, the range of a safe operating area is extended, and protection from overcurrent and overheat can be provided in the wide safe operating area (see, for example, Japanese Patent Application Laid-open No. 2002-280886).
However, in the conventional technology, the safe operating area of a semiconductor device does not conform to the actual characteristics of the allowable power dissipation of the semiconductor device, and, even within the safe operating area, there is an area in which the protective circuits operate and thus the semiconductor device cannot be used.